Electrical parameters measured with time-varying signals are referred to as dynamic parameters. The present invention relates to measuring dynamic parameters of electrochemical cells and batteries through Kelvin (4-wire) connecting cables. More specifically, it relates to suppressing high-frequency (hf) waves oscillating back and forth on a Kelvin cable's current-carrying and/or voltage-sensing conductors.
Measuring automotive and standby cell/battery parameters with time-varying signals (i.e., measuring dynamic parameters) are now commonly accepted maintenance and diagnostic procedures. (See, e.g., U.S. Pat. Nos. 5,140,269, 6,262,563, 6,534,993, and 6,623,314). Because of the very small impedances of such cells/batteries, Kelvin (4-point) connections are routinely employed to reduce the influence of the contact and lead-wire resistances. Kelvin connections couple to each cell/battery terminal at two separate contact points—one for current and one for voltage. Apparatus for measuring a two-terminal cell/battery by means of Kelvin connections therefore requires a four-wire interconnecting cable.
When using Kelvin cables with time-varying signals, distributed mutual-inductance between current-carrying and voltage-sensing conductors has been a problem. As disclosed in U.S. Pat. Nos. 7,106,070 and 7,425,833, mutual-inductance can be reduced by inserting a special cable section in tandem with the original Kelvin cable. This special section transposes conductors thereby introducing a negative mutual-inductance section to cancel the positive mutual-inductance of the original Kelvin cable.
However, even after canceling a cable's mutual-inductance, a significant problem remains. The current-carrying conductors and the voltage-sensing conductors comprise two twisted-pair distributed-parameter transmission lines—not unlike those found in Category-5 Ethernet cables. These lines may extend over several meters in length. As with all distributed-parameter transmission lines, internal wave reflections can occur unless the lines are terminated in their characteristic impedances—a situation which virtually never occurs in practice. Such hf waves, oscillating back and forth, can interact with measuring circuitry to seriously degrade the accuracy of low-frequency dynamic measurements performed with circuitry connected through the Kelvin cables. Ironically, the very technique for reducing mutual-inductance described above introduces discontinuities that can actually contribute to such oscillations. Solving this previously-unrecognized wave-oscillation problem is the purpose of the present invention.
Consider FIG. 1. FIG. 1 depicts prior-art measuring circuitry 10 connected to cell/battery 20 by means of four-wire cable 30, Y-junction 40, and Kelvin conductors A, B, C, and D. Current-carrying conductors A and B couple to positive and negative cell/battery terminals at contact points 50 and 60, respectively. Voltage-sensing conductors C and D separately couple to positive and negative cell/battery terminals at contact points 70 and 80, respectively. During dynamic measurements, a time-varying current flows through current-carrying conductors A and B and also flows inside cell/battery 20 along an internal current path 90.
FIG. 2 shows a typical arrangement of conductors employed in prior-art apparatus such as that shown in FIG. 1. Measuring circuitry 10 comprises current-excitation circuitry 160, voltage-sensing circuitry 170, computation/control circuitry 180, and display circuitry 190. Current-excitation circuitry 160 and voltage-sensing circuitry 170 couple, respectively, to the A-B conductor-pair 140 of four-wire cable 30 at terminals 200 and 210, and to the C-D conductor-pair 150 of four-wire cable 30 at terminals 220 and 230. Computation/control circuitry 180 communicates bilaterally with both current-excitation circuitry 160 and voltage-sensing circuitry 170 and receives current- and voltage-signal inputs with which it computes dynamic parameters of cell/battery 20. The results of this computation are communicated to the user through display 190.
FIG. 2 further discloses a spaced-apart cable section 35 comprising an A-C pair of insulated wires 120 contacting the positive terminal of cell/battery 20 at points 50 and 70, respectively, and a B-D pair of insulated wires 130 contacting the negative cell/battery terminal at points 60 and 80, respectively. Each of these conductor-pairs comprises a current-carrying conductor paired with a voltage-sensing conductor. Pairs 120 and 130 are necessarily spaced-apart at the cell/battery terminals but are brought into close proximity at Y-junction 40 where they are re-arranged for connection to four-wire cable section 30. Throughout section 30, the A-B current-carrying conductors and the C-D voltage-sensing conductors are separately paired and twisted together, pair 140 and pair 150, respectively, to reduce mutual inductance between current-carrying and voltage-sensing circuits. The A-B and C-D conductors therefore comprise two twisted-pair distributed-parameter transmission lines of approximate length l.
FIG. 3 shows current-excitation circuitry 160 of a type commonly employed in prior-art dynamic battery testing apparatus. Feedback excitation circuitry of this kind was first described by Wurst, et al., in U.S. Pat. No. 5,047,722. However, this early disclosure did not include Kelvin connections to the cell/battery, nor did it take into consideration the effect of the distance between the measuring circuitry and the cell/battery being tested.
The A-B current-carrying conductors 360 of the battery-connecting cable are shown in FIG. 3. These conductors include twisted-pair 140 of section 30 as well as the A and B conductors of spaced-apart section 35 of FIG. 2. They may also include a mutual-inductance-canceling section, and their total length can extend several meters.
The current-excitation circuitry 160 disclosed in FIG. 3 comprises the series combination of resistor 300, n-channel MOSFET 310, and the A and B battery-cable terminals, 200 and 210, respectively. This circuitry also includes operational amplifier 320 having its output terminal coupled to the gate of MOSFET 310 through resistor 350. The common connection of resistor 300 and MOSFET 310 couples to the inverting (−) input of operational amplifier 320 through resistor 330, thus providing negative feedback to amplifier 320. As a result, the instantaneous voltage at the amplifier's inverting (−) input, R300 x i(t), tracks the voltage v(t) applied to its non-inverting (+) input. Accordingly, computation/control circuitry 180 controls the current waveform i(t) flowing through cell/battery 10 by applying an appropriate voltage signal v(t) to the noninverting (+) input of amplifier 320. Resistors 330, 350, and capacitor 340 are compensation components—introduced specifically to ensure circuit stability at high frequency.
Note that current i(t) can only pass through n-channel MOSFET 310 from drain to source. Accordingly, MOSFET 310 cuts off, and no current flows through cell/battery 20, when v(t)<0. Cell/battery current can only flow when v(t)>0; and it can then only flow in the discharging direction.
Similar feedback current-excitation circuitry, disclosed in U.S. Pat. Nos. 6,466,026 and 6,621,272, includes a p-channel MOSFET and a dc power supply. With that circuitry, v(t)<0 causes the p-channel MOSFET to conduct—resulting in current flowing from the dc power supply into cell/battery 10 in the charging direction. Thus, cell/battery current can flow in either direction with the advanced circuitry disclosed in U.S. Pat. Nos. 6,466,026 and 6,621,272. In other respects, that circuitry functions just like the circuitry of FIG. 3.
FIG. 4 shows a voltage waveform sometimes observed across series-resistor 300 in prior-art current-excitation circuitry 160 when it is exciting cell/battery 20 with a 22 Hz square wave. One notes large hf oscillations in the A-B current during conduction of MOSFET 310. Close observations have shown that the frequency of these oscillations is greater than 10 MHz. Furthermore, the usual techniques for suppressing hf oscillations in feedback circuits, such as introducing compensation components 330, 340, and 350, or placing picofarad-size bypass capacitors at various points within the circuit, have proven to be surprisingly ineffective. Suppressing such oscillations is an object of the present invention.